Collections in C# standardize the way of which the objects are handled by your program. In other words, it contains a set of classes to contain elements in a generalized manner. With the help of collections, the user can perform several operations on objects like the store, update, delete, retrieve, search, sort etc.

C# divide collection in several classes, some of the common classes are shown below:

System.Collections.Generic Classes

Generic collection in C# is defined in System.Collection.Generic namespace. It provides a generic implementation of standard data structure like linked lists, stacks, queues, and dictionaries. These collections are type-safe because they are generic means only those items that are type-compatible with the type of the collection can be stored in a generic collection, it eliminates accidental type mismatches. Generic collections are defined by the set of interfaces and classes. Below table contains the frequently used classes of the System.Collections.Generic namespace:

Example:

// C# program to illustrate the concept 
// of generic collection using List<T>
using System;
using System.Collections.Generic;
  
class Tech {
  
    // Main Method
    public static void Main(String[] args)
    {
  
        // Creating a List of integers
        List<int> mylist = new List<int>();
  
        // adding items in mylist
        for (int j = 5; j < 10; j++) {
            mylist.Add(j * 3);
        }
  
        // Displaying items of mylist
        // by using foreach loop
        foreach(int items in mylist)
        {
            Console.WriteLine(items);
        }
    }
}

Output:

15
18
21
24
27

System.Collections Classes

Non-Generic collection in C# is defined in System.Collections namespace. It is a general-purpose data structure that works on object references, so it can handle any type of object, but not in a safe-type manner. Non-generic collections are defined by the set of interfaces and classes. Below table contains the frequently used classes of the System.Collections namespace:

Example:

// C# to illustrate the concept
// of non-generic collection using Queue
using System;
using System.Collections;
  
class TAS {
  
    // Driver code
    public static void Main()
    {
  
        // Creating a Queue
        Queue myQueue = new Queue();
  
        // Inserting the elements into the Queue
        myQueue.Enqueue("C#");
        myQueue.Enqueue("PHP");
        myQueue.Enqueue("Perl");
        myQueue.Enqueue("Java");
        myQueue.Enqueue("C");
  
        // Displaying the count of elements
        // contained in the Queue
        Console.Write("Total number of elements present in the Queue are: ");
  
        Console.WriteLine(myQueue.Count);
  
        // Displaying the beginning element of Queue
        Console.WriteLine("Beginning Item is: " + myQueue.Peek());
    }
}

Output:

Total number of elements present in the Queue are: 5
Beginning Item is: C#

Note: C# also provides some specialized collection that is optimized to work on a specific type of data type and the specialized collection are found in System.Collections.Specialized namespace.

System.Collections.Concurrent

It came in .NET Framework Version 4 and onwards. It provides various threads-safe collection classes that are used in the place of the corresponding types in the System.Collections and System.Collections.Generic namespaces, when multiple threads are accessing the collection simultaneously. The classes present in this collection are: